(1) Field of the Invention
This invention relates to an improved process and apparatus for gas enrichment, and more particularly to an improved process and apparatus for gas enrichment using pressure swing adsorption techniques.
(2) Description of the Prior Art
The uses of adsorption techniques to separate a gaseous component from a gaseous stream initially were developed for the removal of carbon dioxide and water from air. The principles of gas adsorption were further refined to processes for enrichment of gases, such as hydrogen, helium, argon, carbon monoxide, carbon dioxide, nitrous oxide, oxygen and nitrogen. Still further refinements using at least two adsorption vessels in a cycling pressurized relationship resulted in an adsorption technique for gas enrichment, commonly referred to as pressure swing adsorption (PSA).
A typical PSA process and apparatus for producing a gas, such as nitrogen from air, employs two adsorption beds, each subjected to four distinct processing steps in each cycle. In a first step of the cycle, one adsorption bed is pressurized with concomitant nitrogen production while the other bed is regenerated, such as by venting, sometimes with a countercurrent flow of product-quality gas to enhance the regeneration (referred to as "purge"). In a second step, sometimes referred to as pressure equalization, the adsorption beds are brought to an equalized pressure by interconnection of the adsorption beds. In a third step of the cycle, the first adsorption bed is regenerated, while the second bed is pressurized with concomitant oxygen production. The last step of the cycle is pressure equalization between the beds.
During such pressure swings, pressure conditions in the adsorption beds vary between 15 psia and 120 psig in a process employing carbon molecular sieves for nitrogen production and somewhat lower pressure ranges in a process employing crystalline zeolites for producing oxygen. While such processes and apparatus have undergone significant refinements, significant energy losses are inherent in such gas enrichment processes as well as the fact that adsorption is itself an exothermic process whereas desorption is an endothermic process. Adsorption is therefore limited by the self-induced temperature rise which reduces adsorption capacity while desorption is slowed by the self-induced fall in temperature. Moreover, in large plants where radial heat transfer is limited, under certain conditions and with certain process parameters, it is possibel for thermal accumulations to produce permanent hot and cold zones which, if severe, can lead to process failure.